a) Field of the invention
The present invention relates to an imaging optical system to be used in television cameras, electronic still cameras, electronic endoscopes and so on.
b) Description of the prior art
In the optical instruments such as television cameras, electronic still cameras and electronic endoscopes, which are adapted so as to obtain colored images by using solid-state image pickup devices or image pickup tubes, image quality is degraded remarkably by spurious signals such as moire and aliasing which are produced due to interference between the sampling frequency which are determined by arrangement of the picture elements in the solid-state image pickup devices and arrangement pitch of the filter elements in the color-coding filter array provided before the solid-state image pickup devices or image pickup tubes, and the spatial frequency components of the images of objects formed on the light receiving surfaces of the solid-state image pickup devices or image pickup tubes.
As a conventional imaging optical system adapted so as to eliminate the spurious signals, there is known the optical system disclosed by Japanese Patent Kokoku Publication No. Sho 51-14033 (U.S. Pat. No. 4,720,637). This imaging optical system comprises an optical low-pass filter composed of a birefringent plate such as calcite.
When imaging condition is changed in photographing an object having a large value at a specific spatial frequency, however, the imaging optical system comprising the optical low-pass filter can not eliminate the spurious signals sufficiently or at all. This defect becomes especially remarkable when a television camera is attached to an eyepiece lens of a fiber scope or when a photograph printed in dots is photographed with a television camera. This imaging optical system will be described detailedly below taking an example wherein the imaging optical system is used in a fiber scope.
A condition where a television camera is attached to the eyepiece of a fiber scope is schematically shown in FIG. 1, wherein a television camera 7 comprising an photographic lens 4, an optical low-pass filter 5 and an image pickup device 6 such as CCD is attached to a fiber scope 3 comprising an image guide fiber bundle 1 and an eyepiece lens 2 so that an image formed on the end surface of emergence of the image guide fiber bundle 1 is focused on the light receiving surface of the image pickup device 6 by the eyepiece lens 2 and the photographic lens 4.
The image guide fiber bundle is composed, as is known well to those skilled in the art, of a large number of optical fibers bundled densely in hexagonal sectional shape. On the end surface of emergence of the image guide fiber bundle, only cores 8 of the fibers arranged regularly as shown on an enlarged scale in FIG. 2 are bright. Accordingly, an image of the end surface of emergence can be considered as an arrangement of the bright spots of the cores 8 which is modulated by distribution of brightness on the object. A spatial frequency spectrum of the image of the end surface of emergence of the image guide fiber bundle has an intense spectrum component at the fundamental frequency determined dependently on arrangement of the cores. The spurious signals are produced due to the interference between the fundamental frequency and the sampling frequency of the CCD 6, and are eliminated insufficiently when the image is photographed with a television camera to which various types of fiber scopes are attached.
FIG. 3A and FIG. 3B show sectional views illustrating a portion of the imaging optical system composed by attaching a television camera to an eyepiece lens of a fiber scope. In these drawings, the reference numeral 1 represents an image guide fiber bundle of the fiber scope, the reference numeral 2 designates an eyepiece lens, the reference numeral 14 (14') denotes an exit pupil of the eyepiece lens, the reference numeral 4 represents a photographic lens of the television camera and the reference numeral 6 designates an image pickup device. An image of object appearing on the end surface of emergence of the image guide fiber bundle 1 is focused on the image pickup device 6 by an imaging optical system consisting of the eyepiece lens 2 and the photographic lens 4. Since NA of each of the rays emerging from the fibers of the image guide fiber bundle 1 of the fiber scope is constant, F number of the imaging optical system is deterimined by the eyepiece lens 4 and size of the exit pupil 14 (14').
Thickness of fiber scopes is various, i.e., thick or thin, dependently on purposes of application. Further, thickness of image guide fiber bundles used in fiber scopes is also various. However, the visual fields visible through eyepiece lenses should have areas which are the substantially the same for convenience of observation. For this reason, eyepiece lenses have various magnifications, i.e., eyepiece lenses provided for thin image guide fiber bundles have high magnifications, whereas eyepiece lenses adopted for thick image guide fiber bundles have low magnifications. In order to observe images through these different fiber scopes at brightness levels which are the substantially the same, diameter of the exit pupil 14 is enlarged so as to thicken the light bundle incident on the photographic lens 4 when the eyepiece lens has a low magnification (or a long focal length) as shown in FIG. 3A, whereas diameter of the exit pupil 14' is reduced so as to thin the light bundle incident on the photographic lens 4 when the eyepiece lens 2 has a high magnification as shown in FIG. 3B since the NA of each of the rays emerging from the optical fibers is constant. Accordingly, the photographic lens 4 has a small F number when the eyepiece lens 2 has a low magnification or a large F number when the eyepiece lens 2 has a high magnification.
Now, let us assume that the optical fibers composing the image guide have the same diameter, that the optical fibers are arranged at a pitch of p, that the eyepiece lens having the low magnification and the photographic lens have a total magnification of .beta..sub.L, and that the eyepiece lens having the high magnification and the photographic lens have a total magnification of .beta..sub.H. Then, mesh-like patterns formed by the optical fibers of the image guide are projected to the light receiving surface of the image pickup device at a high frequency of 1/(p .beta..sub.L) in the case where the eyepiece lens having the low magnification is used, whereas the patterns are projected at a low frequency of 1/(p .beta..sub.H) in the case where the eye-piece lens having the high magnification is used. Accordingly, the fundamental frequency of an image of object is high when an image formed by an endoscope equipped with the eyepiece lens having the low magnification is photographed, i.e., when the imaging optical system has a large F number, whereas the fundamental frequency of an image of object is low when an image formed by an endoscope equipped with the eyepiece lens having the high magnification is photographed, i.e., when the imaging optical system has a small F number.
The optical low pass filter prevents the interference between the spatial frequency components of an image of object and the sampling frequency of an image pickup device by lowering resolution of the image at frequencies higher than a specific spatial frequency. When MTF (Modulation Transfer Function) is taken as the ordinate and spatial frequency is taken as the abscissa, the optical low pass filter has such a frequency response as shown by the solid line in FIG. 12. As is seen from the frequency response curve shown in FIG. 12, it is possible to obtain an effect to eliminate the spurious signals in the imaging optical system having a small F number by using an optical low pass filter which is composed so as to zero MTF at the fundamental frequency 1/(p .beta..sub.L) (the point A in FIG. 12) of the imaging optical system having the small F number. However, it is impossible to lower resolution and eliminate the spurious signals by using the optical low pass filter having the composition described above in the imaging optical system having a large F number since the MTF has a large value at the fundamental frequency 1/(p .beta..sub.M) (the point B in FIG. 12) of the imaging optical system having the large F number. When a low pass filter which zeroes the MTF at the frequency 1/(p .beta..sub.M) is used in the imaging optical system having the large F number so that the spurious signals can be eliminated in the imaging optical system having the large F number, the MTF has a small value at the frequency 1/(p .beta..sub.H) though it should desirably have large values at the frequencies lower than 1/(p .beta..sub.L) and resolution is becomes lower than required, thereby degrading image quality.
As is understood from the foregoing description, the conventional imaging optical system using the optical low pass filter poses various problems for eliminating the spurious signals when the optical system is used for imaging an object which has a large spectrum component at a specific spatial frequency.
Certain imaging optical systems utilize defocusing as a means to eliminate the spurious signals such as moire. Speaking concretely, the spurious signals are eliminated by defocusing images with the imaging optical systems set so as to deviate the image surfaces a little from the optimum positions thereof. When the images are defocused for eliminating the spurious signals at such a degree as to lower the spatial frequency response of the image pickup device to the Nyguist frequency or the spatial frequency response of the image guide fiber bundle to the fundamental frequency described above, it is possible to suppress the high order spectra produced due to the images of the regular arrangement on the end surface of emergence of the image guide and eliminate moire by sampling the images having transmitted through the optical fibers for individual picture elements of the image pickup device.
However, since the ordinary optical systems have rather high response even within the region of high frequencies, the images must be defocused at high degrees for reducing the spatial frequency components and the defocusing of the images linearly lowers the MTF curve as the spatial frequency becomes higher. Accordingly, selection of a low cut-off frequency for elimination of moire results in lowering of response at the frequencies lower than the cut-off frequency and degradation of resolution. Further, moire is not eliminated sufficiently when a measure is taken to prevent resolution from being degraded.
Furthermore, it is necessary to determine degrees of the defocusing in conjunction with apertures of endoscopes to be used with the imaging optical system.